A shiny, silvery, highly radioactive metal found in U and Th ores where it forms as a transient decay product. Formerly used to make luminous paint.

Luminous dial clock.
The luminous paint on the hands and numerals of this mid-20th Century alarm clock contain a small quantity of radium bromide mixed with zinc sulfide. The latter is a phosphor, converting the energy in the alpha particles emitted by the decaying radium atoms to visible light. For this reason luminous paint containing radium does not need to be "charged up" by exposure to light as do modern clock dials. The half life of radium is 1600 years which means the paint will continue to glow for many centuries. Unfortunately it will also continue to produce dangerous radon gas, which is one of several reasons radium's application in luminous paint has ceased.Source: eBaySize: 5 inchesPurity: n/a